Peak detection techniques are required in various applications that require the information of the signal strength or power level. For example, in physical layer designs, particularly in designs with 100 Base TX receivers, peak detection plays an important role in the process of recovering data that has propagated across a medium such as a CAT-5 cable. In some architectures, an average peak voltage level of an input signal is used to determine the length of the cable, and is also used to define the level of equalization required to compensate for amplitude loss and phase distortion incurred by the signal after transmitting along the cable line. The design of peak detectors with good noise immunity becomes more important for systems with smaller incoming signal levels (due to, for example, long cable lengths and/or reduced supply voltage ranges), and for systems with a higher level of chip integration (analog and digital).
FIG. 1A illustrates a block diagram of a conventional peak detector 100 which includes a comparator 105, charge pump 110, and a capacitor 115. The output of the comparator 105 is coupled to charge pump 110 including a controlled pull-up current source 112 for generating current I.sub.1, a controlled pull-down current source 114 for generating current I.sub.2. The output of the charge pump 110 is coupled to the capacitor 115 as well as the negative input terminal "-" of the comparator 105, thus forming a unity gain feedback configuration.
The charge pump 110 and the capacitor 115 generate an average peak voltage signal V.sub.0 across capacitor 115. To detect the peak voltage signal for positive pulses in a data signal, the pull-up current source 112 is much greater than the pull-down current source 114 (i.e., I.sub.1 &gt;&gt;I.sub.2). On the other hand, for negative pulses in a data signal, the pull-down current source 114 is much greater than the pull-up current source 112 (i.e., I.sub.1 &lt;&lt;I.sub.2). As depicted by FIG. 1B, the principle of this conventional peak detector 100 is as follows. The positive pulses peak detection is used as an example. After the average peak level V.sub.0 is achieved, the total area of data signal 150 which is above the level V.sub.0 is denoted as A.sub.1. The total area of data signal 150 which is below the level V.sub.0 is denoted as A.sub.2. The average peak level V.sub.0 is derived to include A.sub.1 ( x )=A.sub.2, wherein x=I.sub.1 /I.sub.2. The ratio of the pump up current I.sub.1 over the pump down current I.sub.2 (or x) is much great than one (1). Similarly, for a negative pulses peak detection, the following is satisfied: A.sub.1 =A.sub.2 ( x ), wherein x=I.sub.2 /I.sub.1. The ratio of the pump down current I.sub.2 over the pump up current I.sub.1 (or x) is much great than one (1).
Conventional peak detectors suffer from various problems and drawbacks such as, for example, data dependency, high sensitivity to noise, and level fluctuation, as discussed below. The data dependent nature of conventional peak detectors is shown in the example of FIG. 1B. Assume that an input data signal 150 is received at the positive input terminal "+" of the comparator 105. Since the data input signal 150 has a dense pulse pattern (i.e., logic high occurs more frequently than logic low), the level of the average peak voltage signal V.sub.0 will be close to the peak level 155 of the input data signal 150 pulses. In contrast, for a data input signal 160 with a sparse pulse pattern (i.e., logic low occurs more frequently than logic high), the level of the average peak voltage signal V.sub.0 is significantly less than the peak level 165 of the input data signal 160 pulses. The average peak voltage signal V.sub.0 tends to drift downward toward the logic low level due to the sparse pulse pattern, and, as a result, may not provide a correct measurement of the peak level 165 of the input data signal 160. To reduce the data-dependent nature of conventional peak detectors, the current ratio provided by current sources 112 and 114 (FIG. 1A) must be adjusted. For example, to detect the peak of positive pulses in a data signal, the ratio of the pull-up current source 112 over its pull-down current source 114 is set at a much higher value (i.e., x=I.sub.1 /I.sub.2 &gt;&gt;1). Thus, even if a sparse pulse pattern signal occurs, the average peak voltage signal V.sub.0 will quickly pull-up to the pulse peak in the data signal.
However, the much higher ratio between the current sources 112 and 114 causes a conventional peak detector to be more sensitive to noise induced at the peak detector input. For example, in FIG. 1C noise 170 may occur at a pulse peak of an input data signal 175. The average peak voltage signal V.sub.0 will quickly rise to at least the noise 170 level. Since the charge rate of current source 112 is much higher than the discharge rate of current source 114, the average peak voltage signal V.sub.0 requires significant time before decreasing to the correct pulse peak level 180. This characteristic makes the average peak voltage signal V.sub.0 very sensitive to the induced noise.
Conventional peak detectors also suffer from a level fluctuation problem that occurs when the peak detector tries to overcome a change in the pulse peak level, as described below with reference to FIGS. 1D and 1E. Conventional peak detectors typically use a drooping mechanism for tracking pulses as the pulses gradually decrease in amplitude. In the case of detecting the peak of a positive pulse, the droop rate is controlled by pull-down current 114 and the capacitor 115. An average peak voltage signal V.sub.0 generated by a conventional peak detector may "droop" so that the decreasing peak levels 180 of an input data signal 185 are properly tracked. FIG. 1D illustrates how this drooping condition permits the tracking of the decreasing peak amplitude. However, FIG. 1E illustrates the drawback caused by the drooping condition of the average peak voltage signal V.sub.0. The average peak voltage signal V.sub.0 will fluctuate if the peak amplitude of the pulse 185 does not decrease its level in a subsequent pulse. In particular, the average peak voltage signal V.sub.0 will droop between pulse occurrences and then suddenly increase by an amount 190 to the peak level 180 during a subsequent pulse occurrence. This condition results in an undesired signal fluctuation.
Therefore, there is a need for an improved peak detector that overcomes the problems of data dependency, high sensitivity to noise, and undesired level fluctuation.